Rebalanced adiabatic optical polarization splitter

ABSTRACT

A polarization splitter/combiner and method of forming the same includes a first waveguide having a direction of propagation in a first direction. The height of the first waveguide is greater than the width of the first waveguide. A second waveguide is in proximity to the first waveguide and has a direction of propagation substantially parallel to the first direction in an interaction region. The second waveguide includes a first portion having a greater than the width of the first portion and a second portion having a width greater than a height of the second portion.

BACKGROUND

Technical Field

The present invention relates to optical polarization splitters and,more particularly, to polarization splitters operating ontelecommunication wavelengths.

Description of the Related Art

Photonic structures can be fabricated on wafer chips in order to createwafers that operate both in an electronic domain and an optical domain.When an optical fiber is used to input light into a waveguide on a waferchip, care must be taken to properly manage the polarization of light.The orientation of the polarization state in an optical fiber changesrandomly with time. The performance of photonic devices on wafer chipsis very sensitive to the orientation of the polarization state. Hence,the input polarization state must be processed on the wafer chip for itto be re-oriented into the polarization state for which the photonicdevices work the best. To achieve such polarization re-orientation, apolarization splitter and rotator (PSR) is used.

Polarization management is a key technology in integrated photoniccircuits. Two orthogonal polarizations of a signal are separated andtreated separately on-chip. There are several different designs that areused to split the polarizations using on-chip photonic structures andwaveguides, each with distinct disadvantages.

In a first polarization splitter, known as a directional coupler, avertical polarization (denoted as “TM” for the “transverse magnetic”mode in a waveguide) couples more strongly with a splitter waveguidethan a horizontal polarization (denoted as “TE” for the “transverseelectric” mode). By bringing the input waveguide into proximity with thesplitter waveguide, the TM polarization is removed from the inputwaveguide and propagates within the splitter waveguide, while the TEmode continues in the input waveguide. However, these structures have anarrow optical bandwidth, high sensitivity to fabrication imperfections,and obtaining low crosstalk necessitates cascading many directionalcouplers.

A second polarization splitter, known as a grating coupler, introduces asignal in a direction perpendicular to the split outputs through agrating. The TE and TM polarizations are scattered in differentdirections by the grating. This enables vertical coupling to the opticalfiber, but again is limited in optical bandwidth and is sensitive to thegrating dimensions.

A third polarization splitter, shown in a top-down diagram in FIG. 1, isknown as a mode-evolution polarization splitter and uses two waveguidesof differing thickness. A first waveguide 102 has a vertical size thatis relatively larger than that of a second waveguide 104, while having ahorizontal size that is relatively narrower. As the horizontal width ofthe first waveguide 102 is increased, TM polarized light (shown as adashed line) becomes confined there while TE polarized light (shown as asolid line) remains confined in the second waveguide. This structure isrelatively broadband and tolerant to variations in waveguide dimensions.However, when integrated into a microelectronic fabrication process andoperating at the short-end of telecommunication wavelengths (e.g., inthe 1.2 to 1.3 micrometer range), the point at which the TM inputcrosses from the second waveguide 104 to the first waveguide 102 is verysensitive to fabrication variation of waveguide 102 and appears close toa minimum feature size that can be reliably fabricated. For example, thesplitter may not function correctly if the first waveguide 102 is a mere20 nm too large.

SUMMARY

A polarization splitter/combiner includes a first waveguide having adirection of propagation in a first direction, wherein a height of thefirst waveguide is greater than a width of the first waveguide. A secondwaveguide is disposed in proximity to the first waveguide and has adirection of propagation substantially parallel to the first directionin an interaction region. The second waveguide includes a first portionhaving a height greater than its width and a second portion having awidth greater than its height.

A method for forming a polarization splitter/combiner includes forming alower layer of a first and second waveguide on a substrate. An upperlayer of the first and second waveguide is formed on the respectivelower layer of the first and second waveguide. The combined height ofthe lower and upper layer of the first waveguide is greater than a widthof the first waveguide. The upper layer of the second waveguide has asubstantially smaller width than a width of the lower layer of thesecond waveguide.

A method of beam splitting includes modifying an effective index ofrefraction of a first waveguide carrying a first signal having a firstpolarization and a second signal having a second polarization that isorthogonal to the first polarization by introducing a top layer to thefirst waveguide that is narrower than the first waveguide. The firstwaveguide is coupled with a second waveguide having a height that istaller than the first waveguide and a width that is narrower than thefirst waveguide to split the second signal into the second waveguide.The first and second waveguides are decoupled after the polarizationshave split. The top layer of the first waveguide is removed.

A method of beam combining includes modifying an effective index ofrefraction of a first waveguide carrying a first signal having a firstpolarization by introducing a top layer to the first waveguide that isnarrower than the first waveguide. The first waveguide is coupled with asecond waveguide having a height that is taller than the first waveguideand a width that is narrower than the first waveguide to combine asecond signal in the second waveguide with the first signal in the firstwaveguide. The second signal has a second polarization that isorthogonal to the first polarization. The first and second waveguidesare decoupled after the polarizations have combined. The top layer ofthe first waveguide is removed.

These and other features and advantages will become apparent from thefollowing detailed description of illustrative embodiments thereof,which is to be read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The disclosure will provide details in the following description ofpreferred embodiments with reference to the following figures wherein:

FIG. 1 is a diagram of a prior art polarization splitter;

FIG. 2 is a cross-sectional diagram of a polarization splitter inaccordance with the present principles;

FIG. 3 is a top-down diagram of a polarization splitter in accordancewith the present principles;

FIG. 4 is a diagram of a crossover point for a transverse magneticpolarized signal according to thickness of respective waveguides andeffective index of refraction in accordance with the present principles;

FIG. 5 is a block/flow diagram of a method for forming a polarizationsplitter in accordance with the present principles;

FIG. 6 is a cross-sectional diagram of a step in the formation of apolarization splitter in accordance with the present principles;

FIG. 7 is a cross-sectional diagram of a step in the formation of apolarization splitter in accordance with the present principles;

FIG. 8 is a cross-sectional diagram of a step in the formation of apolarization splitter in accordance with the present principles;

FIG. 9 is a cross-sectional diagram of a step in the formation of apolarization splitter in accordance with the present principles;

FIG. 10 is a cross-sectional diagram of a step in the formation of apolarization splitter in accordance with the present principles;

FIG. 11 is a cross-sectional diagram of a step in the formation of apolarization splitter in accordance with the present principles;

FIG. 12 is a block/flow diagram of a method for splitting polarizationsin accordance with the present principles; and

FIG. 13 is a block/flow diagram of a method for combining polarizationsin accordance with the present principles.

DETAILED DESCRIPTION

Embodiments of the present principles provide on-chip polarizationsplitters that use two-layer waveguides to control polarization. Thehorizontal widths of the respective layers of the two waveguides arecontrolled over the length of the interaction region of the waveguidesto enhance splitting of the two polarizations.

Referring now to FIG. 2, a cross-sectional diagram of a polarizationsplitter is shown. A first waveguide 202 and a second waveguide 208 areshown. Each waveguide is formed of two layers, the horizontal dimensionsof which change over the length of the waveguides. A bottom layer,forming portions 206 and 212 of the first and second waveguidesrespectively, may be formed from, e.g., crystalline silicon and may havean exemplary vertical thickness between about 50 nm and about 300 nm,with one specific embodiment having a thickness of about 150 nm. Asecond layer, forming portions 204 and 210 of the first and secondwaveguides respectively, may be formed from, e.g., polycrystallinesilicon and may have an exemplary vertical thickness between about 30 nmand about 300 nm, with one specific embodiment having a thickness ofabout 150 nm. This allows for compatibility with existing manufacturingtechnologies, where the bottom layer is formed as part of asilicon-on-insulator substrate layer and the top layer is formed as partof a gate layer. A dielectric layer 214 is formed from, e.g., siliconnitride over the waveguides 202 and 208 having a vertical thicknessbetween about 1 nm and about 400 nm, with one specific embodiment havinga vertical thickness of about 70 nm. A passivating, planarizing layer216 is formed over the dielectric layer 214. The planarizing layer 216is formed with a dielectric material having a refractive index betweenabout 1.33 and about 1.65 at a vertical thickness between about 1 nm andabout 1,000 nm, with one specific embodiment having a thickness of about500 nm. In one specific example, the planarizing layer 216 is formedfrom borophosphosilicate glass.

The lower portions of the waveguides 206 and 212 are formed in an oxidelayer 218 having an exemplary vertical thickness of about 550 nm toabout 5,300 nm that may be formed on a suitable substrate that mayinclude, e.g., a silicon wafer (not shown). A thin layer of oxidematerial 218 may remain over the lower layers of the waveguides 206 and212, having an exemplary vertical thickness between about 1 m to about10 nm, with one specific embodiment having a vertical thickness of about2 nm. The oxide 218 may also extend upward along the sidewalls of theupper layers 204 and 210 of the waveguides to form oxide walls 220,having an exemplary horizontal width of about 5 nm to about 50 nm, withone specific embodiment having a horizontal width of about 20 nm, andhaving an exemplary vertical thickness of about 25% to about 75% thevertical thickness of the upper layers 204 and 210 of the waveguides.

Referring now to FIG. 3, a top-down diagram of a polarization splitter300 is shown. It should be understood that this structure could also beused as a combiner if the inputs and outputs are reversed. As in FIG. 2,the top and bottom layers of each waveguide are shown, with the additionof the narrow top layer 210. Also shown is how these differentstructures vary in width over the course of their length. A table ofexemplary widths and lengths as illustrated is shown below. Notably thetop layer 210 of the second waveguide 208 is narrower at the beginningand end than at W₂, being about 60 nm (W₆). The top layer 210 of thesecond waveguide is introduced over the bottom layer 212 slowly to avoidrotating modes and causing crosstalk between the polarized signals. Inone exemplary embodiment, it is contemplated that the top layer 210 maybe introduced with an initial gap between it and the bottom layer 212 ofabout 700 nm, with a similar gap when the top layer 210 bends out at theend (Gaps 1 and 2). The first waveguide 202 is itself introducedgradually, with an exemplary bend-in gap of about 700 nm (Gap 5).

TABLE 1 Dimension Exemplary Exemplary of interest size (nm) range (nm)W₁ 360 200-1000 W₂ 80  10-1000 W₃ 60  10-1000 W₄ 120  15-1000 W₅ 180 20-1000 W₆ 60   5-1000 W₇ 120  25-1000 L₁ 30,000  3,000-300,000 L₂24,000  3,000-300,000 L₃ 50,000  500-350,000 L₄ 50,000  500-350,000 L₅73,000  600-400,000 L₆ 158,000  400-400,000 Gap 1 700    1-100,000 Gap 2700    1-100,000 Gap 3 140 5-500 Gap 4 700 >300 Gap 5 700    5-100,000

In an interaction region 302, the width of layer 204 increases from 60nm to 120 nm and then to 180 nm. In this region, the TM polarizationtransfers between waveguides. The presence of the layer 212 stands apartfrom conventional splitters. In the previous understanding of adiabaticpolarization splitters, layer 212 would not be present, following thelogic that the horizontal polarization (TE) would follow the horizontalwaveguide 208 while the vertical polarization (TM) would follow thevertical waveguide 202. In contrast, the present embodiments havesuperior performance with a second waveguide 208 that, while still beingmostly horizontal, still has a vertical component in an “inverse-T”shape, as shown in FIG. 2. It is contemplated that there is a gap ofabout 140 nm (Gap 3) between the first waveguide 202 and the secondwaveguide 208 in the interaction region. After the interaction region(or before, in the case of a combiner), it is contemplated that there isa gap between the first waveguide 202 and the second waveguide 208 ofabout 700 nm (Gap 4).

In some cases the conventional “L” shaped structure is too imbalanced toprovide effective polarization splitting, such that a smooth transitionof the vertical polarization from the horizontal to the verticalwaveguide is not possible. Hence the source waveguide 212 is given avertical component 210 to rebalance the structure.

Referring now to FIG. 4, a diagram of the crossover point for the TMsignal is shown, with the vertical axis representing the effective indexof refraction and the horizontal axis representing the widths innanometers of the top and bottom layers 204 and 206 of the firstwaveguide 202, with the latter width shown in parentheses. The effectiveindex of refraction for a given mode represents how favorable thewaveguide is to propagation in the mode. The curve 402 shows how theeffective index of refraction of the first waveguide 202 changes as thewidths change.

Also shown is the effective index of refraction of the second waveguide208, which has in this region a top layer thickness of 80 nm and abottom layer thickness of 360 nm. This is illustrated as the dotted line404 and is contrasted to a similar line 406 for the conventionalimplementation that lacks the upper layer 210 of the second waveguide.The transition of the polarizations occurs when the effective indices ofrefraction are about equal. As can be seen, the thickness at which thetransition occurs for the present embodiments, shown at the intersectionof lines 402 and 404, occurs at nearly 160 nm. In contrast, thethickness at which the transition occurs at the intersection of lines402 and 406 is much smaller, resulting in device dimensions that arechallenging to fabricate cost-efficiently and a greater sensitivity tosmall (e.g., ˜10 nm) differences in width. The present embodimentstherefore move the cross-over thickness to be substantially above theminimum feature size, thereby making them significantly easier tofabricate and more robust against typical manufacturing variations.

It is to be understood that the present invention will be described interms of a given illustrative architecture having a wafer; however,other architectures, structures, substrate materials and processfeatures and steps may be varied within the scope of the presentinvention.

It will also be understood that when an element such as a layer, regionor substrate is referred to as being “on” or “over” another element, itcan be directly on the other element or intervening elements may also bepresent. In contrast, when an element is referred to as being “directlyon” or “directly over” another element, there are no interveningelements present. It will also be understood that when an element isreferred to as being “connected” or “coupled” to another element, it canbe directly connected or coupled to the other element or interveningelements may be present. In contrast, when an element is referred to asbeing “directly connected” or “directly coupled” to another element,there are no intervening elements present.

A design for an integrated circuit chip may be created in a graphicalcomputer programming language, and stored in a computer storage medium(such as a disk, tape, physical hard drive, or virtual hard drive suchas in a storage access network). If the designer does not fabricatechips or the photolithographic masks used to fabricate chips, thedesigner may transmit the resulting design by physical means (e.g., byproviding a copy of the storage medium storing the design) orelectronically (e.g., through the Internet) to such entities, directlyor indirectly. The stored design is then converted into the appropriateformat (e.g., GDSII) for the fabrication of photolithographic masks,which typically include multiple copies of the chip design in questionthat are to be formed on a wafer. The photolithographic masks areutilized to define areas of the wafer (and/or the layers thereon) to beetched or otherwise processed.

Methods as described herein may be used in the fabrication of integratedcircuit chips. The resulting integrated circuit chips can be distributedby the fabricator in raw wafer form (that is, as a single wafer that hasmultiple unpackaged chips), as a bare die, or in a packaged form. In thelatter case the chip is mounted in a single chip package (such as aplastic carrier, with leads that are affixed to a motherboard or otherhigher level carrier) or in a multichip package (such as a ceramiccarrier that has either or both surface interconnections or buriedinterconnections). In any case the chip is then integrated with otherchips, discrete circuit elements, and/or other signal processing devicesas part of either (a) an intermediate product, such as a motherboard, or(b) an end product. The end product can be any product that includesintegrated circuit chips, ranging from toys and other low-endapplications to advanced computer products having a display, a keyboardor other input device, and a central processor.

Reference in the specification to “one embodiment” or “an embodiment” ofthe present principles, as well as other variations thereof, means thata particular feature, structure, characteristic, and so forth describedin connection with the embodiment is included in at least one embodimentof the present principles. Thus, the appearances of the phrase “in oneembodiment” or “in an embodiment”, as well any other variations,appearing in various places throughout the specification are notnecessarily all referring to the same embodiment.

It is to be appreciated that the use of any of the following “/”,“and/or”, and “at least one of”, for example, in the cases of “A/B”, “Aand/or B” and “at least one of A and B”, is intended to encompass theselection of the first listed option (A) only, or the selection of thesecond listed option (B) only, or the selection of both options (A andB). As a further example, in the cases of “A, B, and/or C” and “at leastone of A, B, and C”, such phrasing is intended to encompass theselection of the first listed option (A) only, or the selection of thesecond listed option (B) only, or the selection of the third listedoption (C) only, or the selection of the first and the second listedoptions (A and B) only, or the selection of the first and third listedoptions (A and C) only, or the selection of the second and third listedoptions (B and C) only, or the selection of all three options (A and Band C). This may be extended, as readily apparent by one of ordinaryskill in this and related arts, for as many items listed.

It should be understood that the present embodiments may be implementedusing any appropriate fabrication technology. It is specificallycontemplated that the method of fabrication described herein isparticularly well suited to integration with common fabricationtechniques and lends itself to electronic/photonic chip integration,with photonic components being manufactured using the same processes aselectronic components. This greatly simplifies manufacturing, loweringthe cost and complexity of the process.

Referring now to FIG. 5, a method of fabricating a polarization splitteris shown. Block 502 forms the bottom layer of the waveguides in a toplayer of a substrate, which may for example be formed from an insulatorsuch as silicon dioxide. The bottom layer is formed using, e.g., asingle-crystal semiconductor layer formed from, e.g., silicon, where thematerial used is selected for its optical properties. Block 502 grows ordeposits this bottom layer of silicon on the insulator substrate. Block504 then etches the bottom layer to form the lower sections of thewaveguides. In the embodiments described above, the lower layers 206 and212 are formed in this fashion. Block 506 deposits insulator between andaround the lower waveguide sections 206 and 212. The insulator materialis deposited to, or slightly above, the top plane of the lower waveguidesections 206 and 212. If the insulator material is deposited to a levelhigher than the top plane of the lower waveguide sections, the thicknessabove the lower waveguide sections may be between about 1 nm and about10 nm.

Block 508 forms the top layer of the waveguides on the bottom layer.This may be accomplished by depositing a polysilicon layer over thebottom layer, using any appropriate method of deposition including,e.g., low-pressure chemical vapor deposition, rapid thermal chemicalvapor deposition, sputtering, or plasma-enhanced chemical vapordeposition to form the structures of the top layers 204 and 210discussed above. The polysilicon layer may be grown in place or may bedeposited as a layer and then etched using, e.g., reactive ion etching.Block 510 forms the nitride layer 214 over the top sections 204 and 210.

Referring now to FIG. 6, a cross-sectional view of a step in forming apolarization splitter is shown. A semiconductor-on-insulator substrateis employed in the embodiment shown, with an insulator 604 being formedon a bulk semiconductor base 602. A semiconductor layer 606 is formed onthe insulator layer. It is specifically contemplated that thesemiconductor layers 602 and 606 are both crystalline silicon, but it isrecognized that they may differ and, in particular, that the topsemiconductor layer 606 may be any appropriate material having suitableoptical properties. It is specifically contemplated that the insulatorlayer 604 is a silicon dioxide or other “buried oxide” material.

Referring now to FIG. 7, a cross-sectional view of a step in forming apolarization splitter is shown. The top semiconductor layer 606 isetched to form two lower-layer waveguides 702 and 704. The waveguides702 and 704 may be formed by any appropriate mechanism, including theformation of a mask followed by a liquid etch or reactive ion etching.The lower-layer waveguides correspond to waveguide portions 206 and 212above. As such, the widths of the lower portions of the waveguides 702and 704 may vary.

Referring now to FIG. 8, a cross-sectional view of a step in forming apolarization splitter is shown. A layer of insulator material 802 isformed around the waveguide sections 702 and 704 and on the substrateinsulator layer 604. This insulator layer 802 may be additional silicondioxide or any other appropriate dielectric material. The insulatorlayer 802 may be formed by any appropriate deposition, includingepitaxial growth, low-pressure chemical vapor deposition, rapid thermalchemical vapor deposition, sputtering, or plasma-enhanced chemical vapordeposition. The insulator layer 802 may be formed to a depth thatexceeds the height of the waveguide sections 702 and 704 and then, forexample, buffed to reduce irregularities and expose the waveguidesections 702 and 704. In an alternative embodiment, a portion ofinsulator material (e.g., about 1 mm to about 10 nm) may be left overthe waveguide sections 702 and 704.

Referring now to FIG. 9, a cross-sectional view of a step in forming apolarization splitter is shown. A layer of semiconductor material 902 isformed over the waveguide sections 702 and 704 and the insulator layer802. It is specifically contemplated that the semiconductor layer 902 ispolysilicon, but it is recognized that any appropriate material havingsuitable optical properties may be used. The semiconductor layer 902 maybe formed by any appropriate deposition, including epitaxial growth,low-pressure chemical vapor deposition, rapid thermal chemical vapordeposition, sputtering, or plasma-enhanced chemical vapor deposition.

Referring now to FIG. 10, a cross-sectional view of a step in forming apolarization splitter is shown. The semiconductor layer 902 is etched toform two upper-layer waveguides 1002 and 1004 formed directly on thelower-layer waveguides 702 and 704. The waveguides 1002 and 1004 may beformed by any appropriate mechanism, including the formation of a maskfollowed by a liquid etch or reactive ion etching. The lower-layerwaveguides correspond to waveguide portions 204 and 210 above. As such,the widths of the upper portions of the waveguides 1002 and 1004 mayvary.

Referring now to FIG. 11, a cross-sectional view of a step in forming apolarization splitter is shown. An insulator layer 1102 is formed overthe upper and lower waveguide portions 702, 704, 1002, and 1004. It isspecifically contemplated that a silicon nitride may be used as theinsulator layer 1102, but it should be recognized that any appropriatehardmask material may be used. The insulator layer 1102 may be formed byany appropriate deposition, including low-pressure chemical vapordeposition, rapid thermal chemical vapor deposition, sputtering, orplasma-enhanced chemical vapor deposition. A further passivating layer1104 may be deposited over the insulator layer 1102 and may be formedfrom, e.g., silicon dioxide or any other suitable insulator.

Referring now to FIG. 12, a method for splitting polarizations is shown.Block 1202 introduces a mixed-mode signal to a splitter 300 using ashort, wide waveguide 212. In one specific embodiment, the input signalis infrared light in a range between about 1200 nm and about 1650 nm,with a preferred range between about 1260 nm and about 1360 nm. Block1204 modifies the effective refractive index of the waveguide byintroducing a narrow second layer 210. This is done gradually to avoidmixing between the modes, which would generate unwanted crosstalk.

Block 1206 couples the waveguide 208 with a tall, narrow waveguide 202.The waveguides are brought close to one another. The width of the tall,narrow waveguide 202 is slowly increased in block 1207. At a given pointin the increasing width of the tall, narrow waveguide 202, it becomesmore favorable to the transmission of the TM mode, the TM mode begins topropagate in that waveguide instead. Coupling takes place over atransmission length where the two waveguides 202 and 208 do not touch.Block 1208 then decouples the two waveguides 202 and 208 after thepolarizations have split, moving them farther apart from one another.Block 1210 then removes the second layer 210 from the short, widewaveguide 212. Two signals leave the splitter 300, with the TM modeleaving in the tall, narrow waveguide 202 and the TE mode leaving in theshort, wide waveguide 208.

Referring now to FIG. 13, a method for combining polarizations is shown.Block 1302 introduces two single-mode signals to a combiner 300, with aTM mode in a tall, narrow waveguide 202 and a TE mode in a short, widewaveguide 212. In one specific embodiment, the input signals areinfrared light in a range between about 1200 nm and about 1650 nm, witha preferred range between about 1260 nm and about 1360 nm. Block 1304modifies the effective index of the short, wide waveguide 212 with anarrow second layer 210.

Block 1306 couples the two waveguides 202 and 208 by bringing them closeto one another without touching. Coupling takes place over atransmission length where the two waveguides 202 and 208 do not touch,during which time the tall, narrow waveguide 202 narrows further inblock 1307. This makes the two-layer waveguide 208 more favorable forpropagation of the TM mode, so the TM mode changes over to the two-layerwaveguide 208. Block 1308 decouples the two waveguides 202 and 208 afterthe polarizations have combined and block 1304 removes the second layer210. Addition and removal of the second layer 210 is done gradually toprevent mixing and crosstalk between the signals.

Having described preferred embodiments of a rebalanced adiabatic opticalpolarization splitter (which are intended to be illustrative and notlimiting), it is noted that modifications and variations can be made bypersons skilled in the art in light of the above teachings. It istherefore to be understood that changes may be made in the particularembodiments disclosed which are within the scope of the invention asoutlined by the appended claims. Having thus described aspects of theinvention, with the details and particularity required by the patentlaws, what is claimed and desired protected by Letters Patent is setforth in the appended claims.

The invention claimed is:
 1. A polarization splitter/combiner,comprising: a first waveguide having a direction of propagation in afirst direction, wherein a height of the first waveguide is greater thana width of the first waveguide; a second waveguide, disposed inproximity to the first waveguide and having a direction of propagationsubstantially parallel to the first direction in an interaction region,said second waveguide comprising: a first portion, wherein a height ofthe first portion is greater than a width of the first portion; a secondportion, wherein a width of the second portion is greater than a heightof the second portion; and a layer of insulator material directlybetween the first and second portion.
 2. The polarization splitter ofclaim 1, wherein the width of the first waveguide increases in theinteraction region.
 3. The polarization splitter of claim 1, wherein thefirst and second waveguides are each formed from two layers.
 4. Thepolarization splitter of claim 3, wherein a bottom layer of the firstand second waveguides is formed from crystalline silicon and a top layerof the first and second waveguides is formed from polysilicon.
 5. Thepolarization splitter of claim 1, wherein the first waveguide comprisesa first portion disposed over a second portion, the first portion havinga width that is smaller than a width of the second portion before theinteraction region and increases to equal the width of the secondportion in the interaction region.
 6. The polarization splitter of claim5, wherein the first portion of the first waveguide has a width that issmaller than the width of the first portion of the second waveguide atone end of the interaction region and a width that is larger than thewidth of the second waveguide at an opposite end of the interactionregion.
 7. The polarization splitter of claim 1, wherein the secondwaveguide carries signals having two modes, one of which moves to thefirst waveguide in the interaction region.
 8. A method for forming apolarization splitter/combiner, comprising: forming a lower layer of afirst and second waveguide on a substrate; forming a dielectric layeraround and over the lower layer of the first and second waveguide;forming an upper layer of the first and second waveguide on therespective lower layer of the first and second waveguide, wherein thecombined height of the lower and upper layer of the first waveguide isgreater than a width of the first waveguide, and wherein the upper layerof the second waveguide has a substantially smaller width than a widthof the lower layer of the second waveguide.
 9. The method of claim 8,wherein a width of the first waveguide increases in the interactionregion.
 10. A polarization splitter/combiner, comprising: a firstwaveguide, the first waveguide having a direction of propagation in afirst direction, wherein a height of the first waveguide is greater thana width of the first waveguide, comprising a bottom layer formed fromcrystalline silicon and a top layer formed from polysilicon; a secondwaveguide formed from two layers, the second waveguide being disposed inproximity to the first waveguide and having a direction of propagationsubstantially parallel to the first direction in an interaction region,said second waveguide comprising: a bottom layer formed from crystallinesilicon; and a top layer formed from polysilicon, wherein the secondwaveguide further comprises a first portion and a second portion, theheight of the first portion being greater than a width of the firstportion and a width of the second portion being greater than a height ofthe second portion.
 11. The polarization splitter of claim 10, whereinthe width of the first waveguide increases in the interaction region.12. The polarization splitter of claim 10, wherein the first portion isin direct contact with the second portion.
 13. The polarization splitterof claim 10, wherein a layer of insulator material is directly betweenthe first and second portion.
 14. The polarization splitter of claim 10,wherein the first and second waveguides are each formed from two layers.15. The polarization splitter of claim 14, wherein a bottom layer of thefirst and second waveguides is formed from crystalline silicon and a toplayer of the first and second waveguides is formed from polysilicon. 16.The polarization splitter of claim 10, wherein the first waveguidecomprises a first portion disposed over a second portion, the firstportion having a width that is smaller than a width of the secondportion before the interaction region and increases to equal the widthof the second portion in the interaction region.
 17. The polarizationsplitter of claim 16, wherein the first portion of the first waveguidehas a width that is smaller than the width of the first portion of thesecond waveguide at one end of the interaction region and a width thatis larger than the width of the second waveguide at an opposite end ofthe interaction region.
 18. A polarization splitter/combiner,comprising: a first waveguide having a direction of propagation in afirst direction, wherein a height of the first waveguide is greater thana width of the first waveguide, said first waveguide comprising: a firstportion; and a second portion disposed under the first portion, thefirst portion having a width that is smaller than a width of the secondportion before the interaction region and increases to equal the widthof the second portion in the interaction region; a second waveguide,disposed in proximity to the first waveguide and having a direction ofpropagation substantially parallel to the first direction in aninteraction region, said second waveguide comprising: a first portion,wherein a height of the first portion is greater than a width of thefirst portion; and a second portion, wherein a width of the secondportion is greater than a height of the second portion.
 19. Thepolarization splitter of claim 18, wherein the first portion of thefirst waveguide has a width that is smaller than the width of the firstportion of the second waveguide at one end of the interaction region anda width that is larger than the width of the second waveguide at anopposite end of the interaction region.